It is known that the wheels of vehicles circulating on roads require frequent balancing operations, consisting in the application to the rim of the wheel of small balancing weights, made of lead or other material, suitable for offsetting the irregular distribution of the weights in the tyre.
To perform this operation, balancing machines are commonly used having a so-called “balancing” horizontal revolving spindle, on which the wheel to be balanced must be positioned integral by means of one or more centring and fastening parts.
The amount of wheel unbalance is determined once the wheel has been made to rotate on the balancing spindle by means of a series of electronic or electromechanical devices, such as force transducers applied along the balancing spindle.
To the measurement of the unbalance must be generally added other characteristic measurements, such as the measurement of the roundness of the wheel, of the eccentricity of the wheel, of the level of wear of the tread or the like.
For this purpose, the use is known of auxiliary detection methods and appliance which allow assessing and measuring all the parameters, different from unbalance, caused by non-uniform weight distributions of the tyre/rim assembly, which can affect the correct rolling of the wheel when mounted on the vehicle being driven on the road.
It is in fact underlined that a banal lack of geometric uniformity of the rim, called lateral or radial run-out, can produce a corresponding lack of geometric uniformity on the tyre and, consequently, the wheel will jump during road rolling even though, by means of the balancing machine, its balancing has been correctly performed.
Furthermore, it should not be forgotten that any build-up of weight inside the tyre can also create lack of uniformity on the tyre structure and, therefore, lack of uniformity of the elastic constant of the tyre, which negatively affects the correct rolling, of the wheel on the road.
Such condition can be detected by means of an indirect method based on the determination of the unbalance of the tyre only due to the above weight build-up after deducting any unbalance attributable to the rim.
This method allows determining any geometric faults on the rim and any weight build-up localised faults on the tyre structure, with, if necessary the two effects being offset by means of a matching operation, wherein the rim fault is placed in geometric opposition with respect to the weight fault of the tyre so as to offset the two effects.
This indirect method, nevertheless, is rather complicated and hard to implement. As an alternative to the indirect method, the use is known of auxiliary appliance fitted on balancing machines wherein an idle roller is placed in contact with the wheel with a constant load and the assessment of the lack of uniformity is made by analysing the movement which the roller performs in the presence of the lack of uniformity itself.
A piece of equipment made this way is shown, e.g., in patent document U.S. Pat. No. 6,581,463.
In such piece of equipment the geometric faults of the wheel and any tyre weight build-ups determine the lack of uniformity of the elastic constant of the elastic system of the wheel, in a radial and/or lateral direction, which consequently cause vibrations, or movements in a radial and/or lateral direction which discharge onto the roller.
Such a situation produces in the auxiliary appliance forces in a radial or lateral direction which can be read by suitable sensors.
In other words, we are able to assess the rigidity, or better the rigid nature, of the tyre structure according to the correlation between the quantity of the applied load and the amount of movement of the roller or of the forces which discharge onto it.
This auxiliary appliance of known type is not without drawbacks.
Once combined in fact with a conventional balancing system and in combination with mechanical or no-contact devices for detecting the lateral/radial run-out of the rim and of the tyre, it allows assessing or predicting the behaviour of the wheel on the road once this finds itself operating in real load and speed conditions but, nevertheless, with a rather high degree of approximation.
The application of a rotating roller on the tyre, in fact, does not allow accurately simulating the future rolling of the wheel on the road and this approximation depends on the radius of the roller or, more in detail, on the ratio between the roller radius and wheel radius.
In the appliances of known type, furthermore, the reading of the radial run-out and of the lateral run-out is not always an easy operation, taking into account the fact that the lack of geometric and weight uniformity of the rim/tyre assembly usually produces both a radial component and a lateral component and the measurement of one can affect the accuracy of the measurement of the other.